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Full text of "The manufacture and properties of iron and steel"

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SEGREGATION AND HOMOGENEITY.                         235
Second, the absolute temperature is so high, when compared with everything with which it comes in contact, that conduction and radiation proceed with excessive rapidity.
Third, in the manufacture of ingots for plates, beams, angles, and other rolled or hammered structural material, the steel is cast in direct contact with a thick iron mold, and the absorption of heat from the outside of the liquid is so rapid that a solid envelope is instantly formed, while the conducting power of this envelope is so great that the heat is continually carried from the interior to the surface.
Fourth, the different substances that compose the steel have so many affinities for each other, and combine in so many ways, that it is a gratuitous hypothesis to assume the existence of a definite carbide, or sulphide, or phosphide of iron, or a carbide, sulphide, or phosphide of manganese.
No matter how high or how low the content of metalloids in the steel, there is always a tendency toward the separation of crystals lower in carbon, sulphur, and phosphorus than the average, so that it is logical to conclude that there is a tendency for pure iron to crystallize, but that this is prevented by the affinity it has for carbon, sulphur, phosphorus,. silicon, manganese and copper. This affinity, in conjunction with the rapid cooling, prevents differentiation until a thick envelope has formed on the outside of the ingot to check the loss of heat. Moreover, the process of segregation is self-corrective to some extent, since with every step in the contamination of the interior liquid there is an increasing tendency to the formation of impure crystals.
The liquid center is not homogeneous, for, as the impurities are eliminated from the solidifying envelope, they form alloys or compounds which are more fusible and of lower specific gravity than the'steel, so that they float on the surface of the interior lake. As the level of the metal sinks during solidification, this scum will be deposited on the walls of the pipe cavity, while the history will end by the solidification of a highly impure mass in the apex qf the inverted cone. When there is only a small proportion of sulphur, or phosphorus, or carbon, their hold is so firm that the iron cannot tear itself away, but in larger proportion the affinity of the surplus is weaker. This will explain why tho tendency to segregation increases with an increase in the content of metalloids. Manganese,